gc-libgcrypt: fix undefined enum type in switch statement Resolve the following build failure: lib/gc-libgcrypt.c: In function 'gc_hash_open': lib/gc-libgcrypt.c:317:5: error: case value '0' not in enumerated type 'Gc_hash_mode {aka enum Gc_hash_mode}' [-Werror=switch] case 0: ^~~~ * lib/gc.h (enum Gc_hash_mode): Add value GC_NULL. * lib/gc-libgcrypt.c (gc_hash_open): Use this enum value instead of 0. Signed-off-by: Jia Zhang <qianyue.zj@alibaba-inc.com>